unfuck sysfs_mount()

new_sb is left uninitialized in case of early failures in kernfs_mount_ns(),
and while IS_ERR(root) is true in all such cases, using IS_ERR(root) || !new_sb
is not a solution - IS_ERR(root) is true in some cases when new_sb is true.

Make sure new_sb is initialized (and matches the reality) in all cases and
fix the condition for dropping kobj reference - we want it done precisely
in those situations where the reference has not been transferred into a new
super_block instance.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/sysfs/mount.c

@@ -25,7 +25,7 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
 {
 	struct dentry *root;
 	void *ns;
-	bool new_sb;
+	bool new_sb = false;
 
 	if (!(flags & SB_KERNMOUNT)) {
 		if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
@@ -35,9 +35,9 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
 	ns = kobj_ns_grab_current(KOBJ_NS_TYPE_NET);
 	root = kernfs_mount_ns(fs_type, flags, sysfs_root,
 				SYSFS_MAGIC, &new_sb, ns);
-	if (IS_ERR(root) || !new_sb)
+	if (!new_sb)
 		kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
-	else if (new_sb)
+	else if (!IS_ERR(root))
 		root->d_sb->s_iflags |= SB_I_USERNS_VISIBLE;
 
 	return root;